1. Field of the Invention
The present invention is directed to a new class of microoptical systems and methods for fabricating such microoptical systems and, more particularly, to a method of fabricating in situ, on a surface, through the use of a lithographic process, a plurality of micro-scale optical elements.
2. Description of the Prior Art
Microoptical systems that combine micro-scale optical elements, such as lenses, mirrors, beamsplitters, apertures, prisms, fiber optics, and optical couplers, have capabilities far greater than can be achieved with a single micro-scale optical element. In particular, microoptical systems with clear aperture of about one millimeter or less are required for a number of applications, including medical and biochemical diagnostics, optical fiber switching, and optical data processing. Microoptical systems for these applications typically require the precise alignment and integration of a plurality of micro-scale optical elements.
One method to produce such a microoptical system is to miniaturize a conventional optical bench. Microoptical systems can be produced by fabricating a micro-scale optical bench upon which a plurality of micro-scale optical elements are subsequently mounted and aligned in discrete mechanical devices. Post-assembly and alignment of such microoptical systems on the microscale to achieve a precise focus, collimation, or other optical function is extremely difficult. Because of the manifold difficulties encountered in the assembly and practical application of conventional microoptical systems, a need exists for a microoptical system wherein a plurality of micro-scale optical elements can be precisely assembled and aligned on a substrate.
The present invention solves this problem for microoptical systems having one or multiple optical axes. Micro-scale optical elements are fabricated monolithically on a common surface by a lithographic process, thereby eliminating the need for post-assembly and alignment of separate micro-scale optical elements on a micro-scale optical bench.
A description of lithographic processes can be found in Chapter 1 of Fundamentals of Microfabrication (Marc Madou, CRC Press, 1997).
According to the present invention, any photo- or charged-particle-beam lithographic process that uses a collimated beam of radiation having an absorption length on the order of several hundred microns in a high contrast resist can produce such a microoptical system. In particular, deep X-ray lithography (DXRL) typically uses a highly collimated beam of high energy X-rays from a synchrotron radiation source to achieve a large depth of focus in a thick layer of X-ray photoresist. Thus, DXRL can produce very steep radiation and photoresist profiles. Typical DXRL-produced microstructures have aspect ratios of 100 or greater, with feature heights of up to about 1 mm and sidewall surface roughness of about 10 nm RMS.
Microoptical systems are fabricated from a layer of photoresist applied to a substrate surface using a variety of lithographic processes. The photoresist layer is patterned, typically using a collimated beam of X-rays, in a manner which codes latent profiles therein. The unwanted portions of the photoresist layer are removed, thereby producing a plurality of micro-scale optical elements, such as lenses, mirrors, apertures, and prisms, that are integral to the common substrate. Optical alignment of better than one micron can be achieved. Microoptical systems with micro-scale optical elements made of optical materials other than photoresist can be fabricated by using the developed photoresist layer to fabricate a mold, within which a desired optical material, such as glass, is cast. The mold is then removed to leave a microoptical system made of the desired optical material.